Starting of turbine engines

ABSTRACT

The turbine engine may be started without the use of start injectors by a method including the steps of initiating rotation of a compressor and turbine wheel by applying an external angular accelerating force thereto, introducing fuel through a minority of the fuel injectors employed at about 5-10% of rated engine speed while continuing the application of the accelerating force, introducing fuel through the remainder of the injectors after ignition of the fuel introduced by the prior step and when the engine reaches 15-25% of rated speed while maintaining injection through the minority of injectors, and thereafter modulating fuel flow through all of the injectors to operate the engine in a desired fashion. Also disclosed is a turbine engine practicing the foregoing method.

FIELD OF THE INVENTION

This invention relates to air breathing turbine engines, and morespecifically, to a method and means for starting such engines.

BACKGROUND OF THE INVENTION

Air breathing turbines typically include, as major components, a turbinewheel coupled to a rotary compressor. A combustor receives compressedair from the compressor as well as fuel from a fuel source and burns thesame to provide hot gasses of combustion to drive the turbine wheel.

Many such systems employ so-called "annular" combustors. Such acombustor includes a somewhat toroidal-shaped combustion chambercentered about the rotational axis of the turbine wheel and there areprovided a plurality of circumferentially or angularly spaced fuelinjectors which inject fuel into the annular combustion space as well asair to support combustion during normal operation.

In the usual case, the injectors are of two different types. One type isa so-called "start" injector and the other type is a so-called "main"injector. Generally speaking, the main injectors will substantiallyoutnumber the start injectors and the start injectors are employed onlyduring the start-up sequence for the turbine. The main injectors are notused to initiate start-up of the turbine but may be employed in laterstages of a start-up operation as well as during normal turbineoperation.

Generally speaking, the start injectors are configured to provide forgood atomization of the fuel. Good atomization at the injection nozzlesis necessary because during start-up, the velocity of compressed airreceived from the compressor is relatively low (because the apparatus isrotating at a relatively low speed). Thus, air velocity cannot beutilized to enhance atomization.

Frequently, to achieve the enhanced atomization of fuel that is requiredat low engine rotational speeds, relatively high volumes of fuel arepumped through those nozzles, nozzles typically providing betteratomization for higher pressure drops across the nozzle and/or higherflow rates. This, however, results in local overfueling which in turncauses combustion inefficiencies and damaging hot streaks, i.e., theformation of undesirable hot spots which may damage the combustor. Thus,conventional starting systems have drawbacks in terms of requiring twodifferent types of injectors (which tends to multiply the total numberof injectors involved and thereby increase expense), requiring arelatively large number of control valves, and being prone to theformation of undesirable hot spots.

The present invention is directed to overcoming one or more of the aboveproblems.

SUMMARY OF THE INVENTION

A principal object of the invention is to provide a new and improvedturbine engine, particularly of a relatively small size. Moreparticularly, it is an object of the invention to provide an improvedmeans for starting such an engine. Another facet of the inventioncontemplates an improved method of starting such an engine.

According to one facet of the invention, there is provided a rotaryturbine wheel along with a rotary compressor coupled to the wheel. Anannular combustor is provided for receiving air from the compressor andfuel from a fuel source, combusting the same and providing gasses ofcombustion to the turbine wheel to drive the same. A plurality of fuelinjectors include fuel injecting nozzles angularly spaced about thecombustor and a fuel pump is provided to pump fuel to the nozzles. Firstand second main fuel valves are provided and the first valve is operableto connect a minority of the injectors to the fuel pump for starting theengine. The first and second valves are further operable to connect allof the injectors to the fuel pump for causing normal operation of theengine. The engine is further characterized by the absence of start fuelinjectors for the combustor.

According to one facet of the invention, the fuel injectors aresubstantially identical.

Ideally, the system includes a control schedule valve and the first andsecond main fuel valves are operable to connect the injectors, asaforesaid, to the fuel pump and to the control schedule valve.

In a highly preferred embodiment of the invention, the injectors allinject fuel into the annular combustor in a generally tangentialdirection. Preferably, air is introduced tangentially as well.

In one embodiment of the invention, the minority of injectors is made upof at least two equally angularly spaced injectors and in oneembodiment, there is an even number of the injectors and the minority ofthe injectors consists of two diametrically opposite injectors.

The invention also contemplates a method of starting a small turbineengine having a turbine wheel, a rotary compressor coupled to theturbine wheel, an annular combustor provided with a plurality ofangularly spaced fuel injectors and characterized by the absence ofstart injectors. The method includes the steps of (a) initiatingrotation of the compressor and the turbine wheel by applying an externalangular accelerating force thereto; (b) at about 5-10% of the ratedengine speed, providing fuel to a minority of the injectors whilecontinuing application of the accelerating force; (c) after ignition ofthe fuel introduced during step (b), and when the engine reaches 15-25%of its rated speed, introducing fuel into the combustor through theremainder of the injectors while maintaining injection through theminority of injectors; and (d) thereafter modulating fuel flow throughall of the injectors to operate the engine in a desired fashion.

In a preferred embodiment, step (b) is performed at approximately 7-8%of rated engine speed and step (c) is initiated at an engine speed onthe order of 20% of rated engine speed.

In a highly preferred embodiment, the introduction of fuel, as well ascombustion air, is in the tangential direction.

Other objects and advantages will become apparent from the followingspecification taken in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic, sectional view of a small turbine enginemade according to the invention;

FIG. 2 is a sectional view taken approximately along the line 2--2 inFIG. 1;

FIG. 3 is an enlarged, fragmentary sectional view of an injector; and

FIG. 4 is a schematic of a fuel distribution system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An exemplary embodiment of a gas turbine made according to the inventionis illustrated in the drawings in the form of a radial flow, airbreathing gas turbine. However, the invention is not limited to radialflow turbines and may have applicability to any form of air breathingturbine having an annular combustor.

The turbine includes a rotary shaft 10 journaled by bearings not shown.Adjacent one end of the shaft 10 is an inlet area 12. The shaft 10mounts a rotor, generally designated 14, which may be of conventionalconstruction. Accordingly, the same includes a plurality of compressorblades 16 adjacent the inlet 12. A compressor blade shroud 18 isprovided in adjacency thereto and just radially outwardly of theradially outer extremities of the compressor blades 18 is a conventionaldiffuser 20.

Oppositely of the compressor blades 16, the rotor 14 has a plurality ofturbine blades 22. Just radially outwardly of the turbine blades 22 isan annular nozzle 24 which is adapted to receive hot gasses ofcombustion from an annular combustor, generally designated 26. Thecompressor system including the blades 16, shroud 18 and diffuser 20delivers compressed air to the annular combustor 26, and via dilutionair passages 27, to the nozzle 24 along with the gasses of combustion.That is to say, hot gasses of combustion from the combustor are directedvia the nozzle 24 against the blades 22 to cause rotation of the rotor,and thus the shaft 10. The latter may be, of course, coupled to somesort of apparatus requiring the performance of useful work.

A turbine blade shroud 28 is interfitted with the combustor 26 to closeoff the flow path from the nozzle 24 and confine the expanding gas tothe area of the turbine blades 22.

The combustor 26 has a generally cylindrical inner wall 32, and agenerally cylindrical outer wall 34. The two are concentric and merge toa necked down area 36 which serves as an outlet from an interior annulus38 of the combustor 26 to the nozzle 24. A third wall 39, generallyconcentric with the walls 32 and 34, extends generally radially tointerconnect the walls 32 and 34 and to further define the annulus 38.

Opposite of the outlet 36 and adjacent the wall 39, the interior annulus38 of the combustor 26 includes a primary combustion zone 40 in whichthe burning of fuel primarily occurs. Other combustion may, in someinstances, occur downstream from the primary combustion area 40 in thedirection of the outlet 36. As mentioned earlier, provision is made forthe injection of dilution air through the passages 27 into the combustor26 downstream of the primary combustion zone 40 to cool the gasses ofcombustion to a temperature suitable for the application to turbineblades 22 via the nozzle 24.

In any event, it will be seen that the primary combustion zone 40 is anannulus or annular space defined by the generally radially inner wall32, the generally radially outer wall 34 and the wall 39.

A further wall 44 is generally concentric to the walls 32 and 34 and islocated radially outwardly of the latter. The wall 44 extends to theoutlet of the diffuser 20 and thus serves to contain and directcompressed air from the compressor system to the combustor 26. Mountedon the wall 44 and extending through the wall 34 are injectors,generally designated 46.

As seen in FIG. 2, according to a preferred embodiment of the invention,there are six of the injectors 46 equally angularly spaced about theaxis of rotation of the shaft 10 which is shown by point 48. Theinjectors 46 extend into the primary combustion zone 40 by means ofaligned apertures 50 and 52 respectively in the walls 34 and 44.

With reference to FIGS. 2 and 3, each injector 46 includes a generallycylindrical housing 54 terminating in a radially inward elbow section56. Opposite the elbow section 56, the housing 54 has a peripheralretaining flange 58 which may be sealed by a gasket 60 against amounting surface 62 on the exterior of the radially outer wall 44.

That part of the housing 54 disposed between the walls 34 and 44 isprovided with one or more openings 64, 66 which open to the spacebetween the walls 34 and 44. Thus, compressed air from the compressormay flow through the holes 64, 66 to the interior 68 of the housing 54.

Within the interior 68 of the housing 54 there is disposed a somewhatJ-shaped tube 70. The radially outer end 72 of the tube 70 is in fluidcommunication with a fuel manifold 74 while the radially inner end 76 isangled to correspond with the elbow section 56 of the housing 54 and tobe centered about a reduced diameter opening 78 therein.

Preferably, a fuel swirler 80 may be located within the tube 70 inproximity to the end 76 which serves as an injector nozzle.

The angle of the elbow section 56 and the end 76 of the tube 70 is suchthat both fuel and air will enter the primary combustion zone 40generally tangentially as indicated schematically by spray patterns,generally designated 82, as shown in FIG. 2. Provision may also be madefor the introduction of dilution air into the periphery of the primarycombustion zone 40 in a tangential direction by the provision of aseries of axial lines of apertures 84 and axially elongated coolingstrips 86 as illustrated in FIG. 3.

The tangential injection of fuel and combustion air via the injectors 46as well as tangential introduction of dilution air as just describedprovides for a high degree of circumferential swirl within the primarycombustion zone 40 and thus is highly desirable, though not absolutelynecessary, in practicing the invention since it minimizes the number ofinjectors required to provide an even distribution of mixed air andfuel. In this respect, the invention lends itself readily to use inrelatively small turbine engines. In such engines, because of theirrelatively small size, fuel injectors have heretofore included verysmall fuel injection passages which are highly subject to clogging.Individual passage size for a given engine can be increased by reducingthe number of injectors, thereby allowing each injector to have a largerfuel passage but this can result in the tendency to develop hot spots.According to the invention, the number of injectors may be minimized inthe exemplary turbine engine because the swirl of burning fuel provideseven temperature distribution throughout the primary combustion zone 40.

As illustrated in FIG. 2, there are six, equally angularly spaced onesof the injectors 46 and they are at locations given alphabeticalcharacters A through F, respectively. A system for providing the samewith fuel is illustrated schematically in FIG. 4 and is seen to includea fuel tank 90 connected to the suction side of a fuel pump 92. A bypassline 94 including a pressure relief valve 96 is provided about the pump92 to return fuel to the inlet side of the pump 92 in the event outletside pressure exceeds a predetermined level.

Downstream of the pump 92 and the bypass line 94 is a control schedulevalve 98 which is operated to control the flow of fuel from the pump 92to the injector system as will be seen. The control schedule valve 98 isoperated according to conventional techniques to modulate the flow offuel, primarily during normal operation of the turbine engine. However,according to the invention, it can be utilized to modulate fuel flowduring start-up as well.

Just downstream of the control schedule valve 98 is a solenoid operatedvalve 100 constituting a first main solenoid valve. The injectors 46 atpositions A and D are connected directly to the downstream side of thevalve 100. As illustrated in FIG. 4, they are provided with internalorifices 102 for conventional purposes. If desired, a solenoid valve 104shown in dotted lines in FIG. 4, may be interposed between the injector46 at position D in the solenoid valve 100.

A second main solenoid valve 106 is connected to the downstream side ofthe first main solenoid valve 100 and the injectors 46 at positions B,C, E and F are all connected to the valve 106.

The system is basically completed by a purge valve 110 which is alsoconnected to the outlet side of the first main solenoid valve 100.

According to a preferred embodiment, each of the injectors 46 at each ofthe positions A-F, inclusive, is identical one to the other. That is tosay, they are all main fuel injectors and it will be appreciated fromthe review of the foregoing description as well as a review of thedrawings, particularly FIG. 2, that the turbine engine is characterizedby the absence of even a single start injector.

In normal operation of the turbine engine, the solenoid valves 100 and106, and the solenoid valve 104 if present, will all be open and fuelinjection will be occurring at each of the positions A-F, inclusive, theamount of fuel being injected being determined by operation of thecontrol schedule valve 98 in a conventional fashion.

To shut down the engine, the first main solenoid valve 100 is closed andthe purge solenoid valve 110 opened to purge all fuel from the system.

In starting the turbine engine, an external accelerating force isapplied to the shaft 10 as by a starter motor (not shown). As the rotorbegins to accelerate, in the range of about 5-10% of rated engine speed,and preferably in the range of about 7-8% of rated engine speed, thefirst main solenoid valve 100 is opened and fuel is injected into theprimary combustion zone at the locations A and D. If the start-up of theengine is to occur at a high altitude as when the turbine is utilized inan aircraft, and if the solenoid valve 104 is included, the same willremain closed so that injection will only occur at area A because of thelow fuel flow rate that is desired for high altitude start-ups.

In any event, generally at about 10% of rated speed, ignition of thefuel thus injected will occur. By a suitable temperature sensor (notshown) placed in the engine downstream of the turbine blades 22, theresulting rise in exhaust temperature can be sensed and throughconventional controls, the second main fuel solenoid valve 106 opened.In a typical case, the resulting injection of fuel at the locations B,C, E and F will occur at an engine speed in the range of about 15-25% ofrated engine speed and more specifically, at an engine speed on theorder of about 20% of rated engine speed. If present, the solenoid valve104 will be opened at this time simultaneously with the second mainsolenoid valve 106 or at some point in time intermediate the opening ofthe first main solenoid valve 100 and the opening of the second mainsolenoid valve 106 as desired.

Acceleration of the shaft will continue to occur and injection at allsix locations A-F inclusive will be modulated for optimum fuel flow byconventional control of the control schedule valve 98; and the enginewill shortly obtain 100% of rated speed.

Thus, the invention provides a means of eliminating any need forspecialized start injectors. By reason of the fact that tangentialinjection of combustion air and fuel allows the use of fewer injectors,a high degree of atomization necessary for starting can nonetheless beobtained through the use of main injectors which may not have theatomizing characteristics of start injectors. In particular, even thoughair flow through the system will be low during the starting of theinvention, thus inhibiting atomization, because only two injectors 46are utilized (and only one injector 46 at high altitude), sufficientfuel flow through the same and/or pressure drop across the samenecessary to achieve the desired high degree of atomization necessaryfor starting can be obtained. Indeed, the two injectors may beoverfueled during start-up in order to obtain ignition without concernfor the creation of hot spots because the tangential flow of fuel andair within the annular combustor 40 evens temperatures throughout. Suchoverfueling can be immediately cut back by appropriate control of thecontrol schedule valve 98 once a rising exhaust gas temperatureindicating that ignition has been obtained is sensed.

What is claimed:
 1. A relatively small turbine engine comprising;arotary turbine wheel; a rotary compressor coupled to said turbine wheel;an annular combustor for receiving air from said compressor and fuelfrom a fuel source, combusting the same and providing gases ofcombustion to said turbine wheel to drive the same; a plurality ofsubstantially identical main fuel injectors including fuel injectingnozzles angularly spaced about said compressor; fuel and air from saidcompressor being introduced into said combustor generally in thetangential direction; a fuel pump; a control schedule valve; and firstand second main fuel solenoid valves; said first valve being operable toconnect a minority of said injectors to said control schedule valve andsaid fuel pump for starting said engine, there being an even number ofsaid injectors and said minority of injectors consisting of twodiametrically opposite injectors; said first and second valves beingoperable to connect all of said injectors to said control schedule valveand said pump for causing normal operation of said engine; said enginefurther being characterized by the absence of start fuel injectors forsaid combustor.